1. Field of the Invention
The present invention relates to a power supply apparatus which operates to regulate an output voltage of an external power source such as a battery at a predetermined voltage value, and supplies it to various power supply objects (electric loads).
2. Description of Related Art
It is common that a vehicle-mounted electronic control unit generates, from a battery voltage (a voltage at a positive terminal of a vehicle battery), a power supply voltage lower than the battery voltage, and supplies it to a microcomputer and its peripheral circuits included therein.
As a power supply apparatus capable of generating such a power supply voltage, a series power supply circuit (a series regulator), or a stepdown switching power supply circuit (a stepdown type switching regulator) can be used. The series power supply circuit can output a voltage with a small ripple, however, the efficiency thereof is low and accordingly power loss thereof is large. On the other hand, the stepdown switching power supply circuit has a high efficiency (small power loss), however, a ripple in its output voltage is large.
Accordingly, there has been proposed a power supply apparatus constituted by a series power supply circuit and a stepdown switching power supply circuit located in the upstream (in the prestage) of the series power supply circuit, in order to reduce the power loss of the series power supply circuit, and reduce the ripple in the output voltage (refer to Japanese Patent Application Laid-open No. 2005-312141, for example).
In the power supply apparatus having such a configuration, since the stepdown switching power supply circuit causes switching noise, a filter circuit having an inductance needs to be provided in the upstream of the stepdown switching power supply circuit, so that the switching noise caused by the stepdown switching power supply circuit is not emitted to the outside of an electronic control unit in which the power supply apparatus is mounted. This makes it possible to prevent electric equipment located near the electronic control unit such as a radio receiver from being affected by the switching noise.
The power supply apparatus having the configuration described above may be provided with a stepup switching power supply circuit (a stepup type switching regulator), so that the electronic control unit can continue its operation even when the battery voltage falls below the power supply voltage to be supplied to various components within the electronic control unit.
FIG. 8 is a circuit diagram showing a structure of an example of such a power supply apparatus. As shown in this figure, this power supply apparatus includes a series power supply circuit 31, a stepdown switching power supply circuit 21, a filter circuit 11, and a stepup switching power supply circuit 41.
The series power supply circuit 31 is constituted by a transistor (PNP transistor in this example) 32, a capacitor 33, and a control circuit 34 operating to control the transistor 32. Two output terminals (emitter and collector) of the transistor 32 are connected in series between a power supply line L3 transmitting an output voltage V3 of the stepdown switching power supply circuit 21 and a power supply line L4 through which a power supply voltage V4 is supplied to various power supply objects including a microcomputer included in the electronic control unit. The capacitor 33 is connected between the power supply line L4 and a ground line (a line at a ground potential to which a negative terminal of a vehicle battery is connected) to suppress ripple and noise in the power supply voltage V4. The control circuit 34 controls a base current of the transistor 32 such that the power supply voltage V4 is regulated at a target voltage (5V, for example).
The series power supply circuit 31 having the above described structure operates to reduce the output voltage of the stepdown switching power supply circuit 21 to generate the power supply voltage V4, and supply it to the power supply objects.
The stepdown switching power supply circuit 21 includes a transistor (P-channel MOSFET in this example) 22, an inductor 23, a diode 24, a capacitor 25, and a control circuit 26 operating to control the transistor 22.
One input terminal (drain) of the transistor 22 is connected to a power supply line L2 transmitting an output voltage V2 of the filter circuit 11 or of the stepup switching power supply circuit 41. The other output terminal (source) of the transistor 22 is connected to the cathode of the diode 24 and to one end of the inductor 23. The anode of the diode 24 is connected to the ground line. The other end of the inductor 23 is connected to the power supply line L3. The capacitor 25 is connected between the power supply line L3 and the ground line.
When the transistor 22 is on/off driven, the voltage outputted from the source of the transistor 22 to the one end of the inductor 23 changes alternately between the voltage V2 of the power supply line L2 and 0V. This output voltage of the transistor 22 is smoothed by a smoothing circuit constituted by the inductor 23, the diode 24, and the capacitor 25, and then applied to the power supply line L3. The control circuit 26 on/off controls the transistor 22 such that the voltage of the power supply line L3 is kept at a constant target voltage higher than the power supply voltage V4. Incidentally, when the transistor 22 changes from the on state to the off state, the diode 24 allows a surge current to flow through the inductor 23.
The filer circuit 11, which is constituted by an inductor 12, and capacitors 13, 14, is series-connected between a power supply line L1 to which a battery voltage V1 as an external power supply voltage is applied, and the power supply line L2. The capacitor 13 is connected between the power supply line L1 (or the upstream end of the inductor 12) and the ground line. The capacitor 14 is connected between the power supply line L2 (or the downstream end of the inductor 12) and the ground line.
The stepup switching power supply circuit 41 is constituted by an inductor 42, a transistor (N-channel MOSFET in this example) 43, a diode 44, a control circuit 45 operating to control the transistor 43, and the capacitor 14.
One end of the inductor 42 is connected to the power supply line L1. Two output terminals (drain and source) of the transistor 43 is connected in series between the other end of the inductor 42 and the ground line. The anode of the diode 44 is connected to a node between the inductor 42 and the transistor 43 (the drain of the transistor 43). The cathode of the diode 44 is connected to the power supply line L2 and to the capacitor 14 (to be more exact, to the terminal of the capacitor 14, which is located on the side opposite to the ground line). The capacitor 14 is also used as a component constituting the filter circuit 14. The above described structure of the stepup switching power supply circuit 41 is disclosed, for example, in Japanese Patent Application Laid-open No. 2005-117784.
When the transistor 43 is turned on, the voltage at the node between the inductor 42 and the transistor 43 becomes about 0V, and when the transistor 43 is turned off, a flyback voltage higher than the voltage V1 of the power supply line L1 appears at this node. Accordingly, when the transistor 43 is on/off driven, the voltage at the node between the inductor 42 and the transistor 43 changes alternately between about 0V and the flyback voltage higher than the voltage V1. This changing voltage is rectified and smoothed by a smoothing circuit constituted by the diode 44, and the capacitor 14, and then applied to the power supply line L2. The diode 44 is for preventing current backflow from the capacitor 14 to the transistor 43 when the transistor 43 is turned on. The control circuit 45 keeps the transistor 43 at the off state when the voltage V1 of the power supply line L1 is higher than a certain value. On the other end, when the voltage V1 of the power supply line L1 falls below this certain value, the control circuit 45 turns on the transistor 43 at such a duty ratio that the voltage of the power supply line L2 (that is, the output voltage of the stepup switching power supply circuit 41, or the input voltage of the stepdown switching power supply circuit 21) is kept at a constant target voltage higher than the power supply voltage V4.
With such a power supply apparatus having the stepup switching power supply circuit 41, it is possible to continue supplying the power supply voltage V4 equal to 5V to the microcomputer etc., even when the battery voltage V1 falls below 5V, because the battery voltage V1 can be stepped up above 5V, and this stepped up battery voltage is supplied to the stepdown switching power supply circuit 21, and thereafter to the series power supply circuit 31. Incidentally, when the transistor 43 is kept at the off state (that is, when the stepup switching power supply circuit 41 is not in operation), the battery voltage V1 of the power supply line L1 is supplied as it is to the stepdown switching power supply circuit 21 through the filter circuit 11, so that the battery voltage V1 is stepped down to the power supply voltage V4 of 5V by the operations of the stepdown switching power supply circuit 21 and the series power supply circuit 31 to be supplied to the microcomputer etc.
There is known a stepup/stepdown type DC/DC converter in which the same inductor is shared between its stepdown switching power supply circuit and its stepup switching power supply circuit. For example, refer to Japanese Patent Application Laid-open No. 2000-166223.
The conventional power supply apparatus as shown in FIG. 8 is large in size and is high in manufacturing cost, because each of the stepdown switching power supply circuit and the stepup switching power supply circuit included therein needs an inductor. This makes it difficult to reduce the size and manufacturing cost of the electronic control unit in which such a conventional power supply apparatus is mounted.
It may occur that one inductor can be eliminated, if the inductor 23 used in the stepdown switching power supply circuit 21 is also used for the stepup switching power supply circuit 41 by utilizing the technique enabling to share the same inductor between the stepdown switching power supply circuit and the stepup switching power supply circuit, as disclosed in Japanese Patent Application Laid-open No. 2000-166223. However, it is not practical to apply such a technique to the conventional power supply apparatus as shown in FIG. 8 for the reasons set forth below.
First, when the transistor 43 of the stepup switching power supply circuit 41 is turned on, not only a current flows through the inductor 23 used for both the voltage stepup operation and the voltage stepdown operation, but also a large current flows through the inductor 12 of the filter circuit 11 and the transistor 22 of the stepdown switching power supply circuit 21 by way of the transistor 43 of the stepup switching power supply circuit 41. Accordingly, the power loss of the entire power supply apparatus increases inadmissibly.
Secondary, to use the inductor 23 for both the voltage stepup operation and the voltage stepdown operation, the number of turns thereof has to be increased, and the winding wire thereof has to be thickened. That is because the inductor needs to have a large inductance to provide a large voltage smoothing effect when it is used for the voltage stepdown operation, and needs to have a small internal resistance to provide a high power supply capacity when it is used for the voltage stepup operation.